Lithium ion secondary batteries have attracted attention because of their high energy density and have been actively researched and developed.
A conventional lithium ion secondary battery often uses a solution prepared by dissolving a lithium salt such as LiPF6 into a mixture solvent of cyclic carbonate such as ethylene carbonate (EC) or propylene carbonate (PC) and chain carbonate such as diethyl carbonate (DEC) or dimethyl carbonate (DMC) as an electrolyte solution. The non-aqueous electrolyte solution prepared by using these carbonates has a low flash point of about 30° C. Therefore, when an electrolyte solution leaks out by some accident in the presence of a fire source, there is a danger of a fire caused by the leaking electrolyte solution which catches fires. In view of this, various techniques of raising a flashpoint of an electrolyte solution have been studied. For example, mixing a flame retardant into an electrolyte solution has been studied as one of techniques of raising a flashpoint of an electrolyte solution (see Patent Document 1, for example).